In the design of solenoids and other electrical load devices, the designer must sometimes account for the variation of resistance with temperature in determining the number of amp-turns required for the device to operate as intended. The problem is further complicated in cases where the available voltage supply varies over some predetermined range in response to changing conditions. Thus, the designer may have to consider both variation in resistance and variation in supplied voltage in determining the required amp-turns for the device.
For example, in an application where a bus voltage varies from 14 to 32 volts and coil resistance varies from 20 to 60 ohms, the designer must ensure that the device can draw from about 0.25 to 1.5 amps. However, the device needs only the minimum level of current (0.25 amps) to ensure that amp-turn requirements are met. So long as that minimum level of current is provided, the device will perform as needed under all operating conditions within the prescribed ranges of voltage and resistance. By driving the device with only the minimum level of required current, the user of the device consumes less power than he would if it were driven in the more conventional manner of allowing output current to vary in accord with power requirements.
An object of the present invention is to provide a load driver which operates at a constant level of output current.
A further object of the invention is to provide such a load driver with fault tolerance, meaning that a failure in the load device will result in zero output voltage.
Other objects and advantages of the invention may become apparent from the following description, which includes the appended claims and accompanying drawing.